What is the counterpart of a consultant?
Someone who is consulted is a consultant, but what is his counterpart called? In other words, what do you call the person who consults them?
Asker sounds informal, consulter doesn't pass my spell check, help-seeker sounds too pathological.
Is there a sensible opposite?
"consultant" is too broad a term.
A consultant (from Latin: consultare "to deliberate") is a professional who provides expert advice[1] in a particular area such as security (electronic or physical), management, education, accountancy, law, regulatory compliance, human resources, marketing (and public relations), finance, health care, engineering, science or any of many other specialized fields. A consultant is usually an expert or an experienced professional in a specific field and has a wide knowledge of the subject matter. from Wikipedia
A medical doctor can be a consultant. In this case, one who is seeking medical advice, would be called "patient".
A lawyer, as well as several other professionals, can also be a consultant. Here, the person seeking help or advice is usually called "client".
patient (noun) - One who receives medical attention, care, or treatment. TFD
client (noun) - The party for which professional services are rendered, as by an attorney. TFD